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High five! Physicists create record ‘fat’ photon

By Colin Barras

Physicists have created a “fat” particle made of a record-breaking five entangled photons in a quantum state with echoes of Schrödinger’s cat. Such photons could be put to use in high-resolution imaging.

The physical properties of entangled particles are intimately linked, even across vast distances. Physicists want to entangle ever-larger numbers of particles to understand why the phenomenon isn’t seen at the macroscopic scale.

It’s already possible to entangle six photons in a so-called graph state – ideal for quantum computing because each particle occupies a unique mode, or route, through a quantum circuit.

But another form of entanglement that is employed in high-resolution imaging has been stalled at three or four photons – until now. In a “NOON” state, if the entangled photons are given a choice of two paths through an optical medium, all will opt to follow the same one. As such, a NOON state is comparable to Erwin Schrödinger’s famous thought experiment, in which a cat can occupy only one of two states – alive or dead – when it is hidden in a box and exposed to a potentially lethal source.

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Now Itai Afek, Oron Ambar and Yaron Silberberg at the Weizmann Institute of Science in Rehovot, Israel, have pushed the number of photons entangled in the NOON state up to a record-breaking five.

Split pulses

They used a laser to generate short pulses of infrared light and split each pulse in two. They then converted one half into a quantum pulse, using a transparent crystal of beta barium borate to cleave some of the photons into entangled pairs that behave in a correlated way. Finally, they combined this quantum pulse with the other half of the pulse, theoretically generating a NOON state containing up to five entangled photons.

To prove the NOON states’ existence and measure how many photons they had entangled, the team sent the photons through a two-path device, and detectors recorded their progress. Entangled photons in a NOON state behave as if they are part of a single bulky photon – a “fat photon”, as Silberberg calls it, which will travel down one of the two paths

A fat photon’s wavelength becomes shorter than normal, and the amount of shortening depends on the number of entangled photons it comprises, Silberberg explains. So by recording fat photons with a wavelength five times shorter than expected, the team demonstrated that they had succeeded in creating a five-photon NOON state.

The fat photons produced this way are of more than just academic curiosity, says Christoph Wildfeuer at the Louisiana State University in Baton Rouge. Because the infrared photons behave as if they have a much shorter ultraviolet wavelength, they could be used in remote sensing to image objects with the greater resolution ultraviolet light affords, but in situations where such light is scattered or absorbed. If penetrating the atmosphere with ultraviolet photons is not possible, says Wildfeuer, entangling infrared photons could do the job instead, with the higher resolution of the ultraviolet photons.